01 - Preheat the oven to 375°F. Grease a 9 x 13 inch baking dish with butter or cooking spray.
02 - Bring a large pot of salted water to a boil. Add the macaroni and cook until al dente, about 6 minutes. Add broccoli florets during the last 2 minutes. Drain well in a colander and set aside.
03 - In a medium saucepan, melt 4 tbsp butter over medium heat. Add flour and whisk constantly for 1-2 minutes, until bubbling and fragrant but not browned.
04 - Gradually whisk in the milk until smooth. Add Dijon mustard, garlic powder, onion powder, pepper, and salt. Cook, stirring constantly, until thickened and just starting to bubble, 5-7 minutes.
05 - Remove from heat. Stir in cheddar and Gruyère cheese until fully melted and smooth. The sauce should coat the back of a spoon.
06 - Combine cooked pasta and broccoli with the cheese sauce in a large bowl. Mix well until evenly coated, then transfer to the prepared baking dish.
07 - In a small bowl, mix breadcrumbs with melted butter and Parmesan cheese until combined. Sprinkle evenly over the macaroni mixture.
08 - Bake for 20-25 minutes, or until the top is golden brown and the sauce is bubbling around the edges.
09 - Let stand 5 minutes before serving to allow the sauce to set slightly for easier serving.